PanzerSajt, check first the image you are trying to mount with

sudo fdisk -l -u ubuntu-11.10-preinstalled-desktop-armel+omap.img

it should give 'Start' for the first partition 1, also notice there fs type
I am not quite sure, but maybe you will need to load some modules for that fs type you have in the image (and if the offset is correct -- 512 should be correct for 'Start' from 1-st sector) (maybe it is necessary to load btrfs or ext4 and jb2, like : sudo modprobe btrfs)